This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A115976 #20 Aug 20 2021 04:21:53
%S A115976 1,3,49737,717027,9723611,21335267,32390921,38999627,43091897,
%T A115976 86071337,101848553,102361457,228911411,302948067,370219467,393664027,
%U A115976 455781089,483464027,1040406177,1272206987,2371678553,2571052241,2648052857,3054713937,3597613307,3782971499,3917903851,4005163577,5419912241
%N A115976 Numbers k that divide 2^(k-2) + 1.
%C A115976 Some larger terms: 4465786944074559659, 1440261542571735083956640176981881665928575750093930787551969
%H A115976 Max Alekseyev, <a href="/A115976/b115976.txt">Table of n, a(n) for n = 1..708</a> (contains all terms below 10^15)
%t A115976 lst = {}; Do[ If[ PowerMod[2, 2n - 3, 2n - 1] == 2n - 2, AppendTo[lst, 2n - 1]], {n, 10^9}]; lst (* _Robert G. Wilson v_, Apr 04 2006 *)
%Y A115976 The odd elements of A244673.
%Y A115976 Cf. A006521, A006517, A069927, A067945, A067946, A067947, A068382, A068383, A014945, A014946, A014949, A092028.
%K A115976 nonn
%O A115976 1,2
%A A115976 _Max Alekseyev_, Mar 15 2006
%E A115976 More terms from _Robert G. Wilson v_, Apr 04 2006
%E A115976 Terms a(24) onward from _Max Alekseyev_, Feb 03 2015
%E A115976 b-file corrected and extended by _Max Alekseyev_, Oct 27 2018